On the SNDDST command there is also a LONGMSG parameter which can handle up to 5000 characters.
The direct answer is that SNDDST is not appropriate for this. It is the wrong tool. It simply isn’t equipped for it.
Use the SMTP facilities that are designed for it. In particular, use the QtmmSendMail API, the java mail API or others, or download and install any of the freeware or commercial products that can do the job.